Asphalt compositions have been finding versatile use in such applications as road paving, waterproof sheets, sound insulation sheets, roofing and the like. In such cases, a number of attempts have been made to improve properties of asphalt by adding various polymers. Regarding illustrative examples of such polymers, ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers, rubber latex, block copolymers comprising butadiene and a vinyl aromatic hydrocarbon and the like are used.
However, ethylene-vinyl acetate copolymers and ethylene-ethyl acrylate copolymers are not desirable, because low temperature characteristics of the resulting asphalt compositions are poor, which causes generation of cracks in winter. In addition, since they have poor ductility characteristics, their caking power (tenacity) is also poor so that they show poor aggregate grasping characteristics in the case of road paving.
In the case of rubber latex, it has economical or process problems such as the necessity to carry out excess heating for the evaporation of water contained in latex.
In recent years, a demand for the maintenance of superior strength and wear resistance has been increasing because of heavy traffic roads and highways due to increasing number of road-passing cars or acceleration of traffic. In addition, another demand has also been increasing for a high performance asphalt composition (an asphalt binder for draining pavement use) for the purpose of improving drainage property and reducing ambient noises in constructing a pavement having high void ratio, so that more high softening point and mechanical strength such as toughness and tenacity have been in demand.
Furthermore, some new problems have been found such as poor storage stability of asphalt compositions after storage over a long period of time. The wording "poor storage stability" as used herein means a phenomenon in which the performance such as softening point wholly reduced, or phase separation in which the performance becomes different between the upper layer and the lower layer, takes place after high temperature storage. Such phenomena have not been solved and posing serious problems. In order to solve these problems, attempts have been made to improve asphalt compositions, for example, by increasing molecular weight of the aforementioned block copolymer. However, though simple increase in the molecular weight can improve mechanical strength, melt viscosity becomes high and causes new problems such as considerable sacrifice of workability in road paving and the like. Regarding the storage stability, increment of molecular weight causes sharp reduction of stability so that its improving effect cannot be found.
As a high performance asphalt composition having more high softening point, an asphalt composition to which a block copolymer having a specific molecular structure was added has been proposed as disclosed in JP-A-6-041439 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). This composition has high softening point, penetration and ductility and excellent cold resistance and workability, but its storage stability is not improved so that it cannot be used when storage stability is required.
Thus, there is no conventional asphalt composition to which various polymers are added and which satisfies high softening point, penetration and ductility and high degree balance with workability, and also has excellent storage stability, so that there is a strong demand for such an asphalt composition.